This invention relates to a device for feeding and adjusting a continuous web and for cutting it into portions, and in particular a web on which is printed a plurality of motifs which have to be centred precisely and always identically on each portion which is cut.
In order to obtain good constant centering of the printed motif on each portion, an adjustment operation has to be carried out before the cutting operation.
In this respect, it is well known that the distance between the centres of printed motifs on a continuous web is not exactly constant for various reasons deriving from the actual printing process, from the variation in the tension to which the web is subjected during its unwinding, and from variations in ambient conditions which can cause it to contract or elongate.
For these reasons, when a web printed on one or both of its faces has to be divided into portions, each cutting operation is of necessity preceded by an operation in which the position of the printed motif is checked, which is followed if necessary by an adjustment operation. By means of these operations, portions are obtained which, although they may not be of precisely constant length, carry on their surface printed motifs which are perfectly centred.
For this purpose, according to the known art, reference marks (colour marks, holes, slots) are provided on the continuous web during its printing and at the same distance apart as the printed motifs, such as to correspond with each length which is to constitute an individual portion. The checking and adjustment operation is therefore carried out using these marks for reference purposes.
Some of the known devices therefore comprise means for feeding the web, constituted by rollers which are driven intermittently to unwind, during each cycle, a length of web which is approximately equal to but greater than the length of one portion. As the reference mark passes through its reading zone, a photoelectric cell provides a signal. If this signal is in synchronism with a second signal generated by a cyclic machine cam in fixed phase relationship with the periodic cutting means, this signifies that it is not necessary to adjust the web before the cutting operation. If said signals do not coincide, then an adjustment operation is necessary.
In these known devices, because of the fact that the cyclic feed is excessive by an amount greater than any variation in the distance between the reference marks, a phase displacement between the two signals occurs after a certain number of cycles starting from a condition of perfect adjustment, and more precisely the photoelectric cell reading signal occurs before the signal provided by the cyclic machine cam. At this point, there is automatic energisation of an electromagnet, which, by way for example of deviation rollers over which the web runs, resets the adjustment by dragging the length of web lying between the cutting device and photoelectric cell through a predetermined fixed distance in the direction opposite the direction in which the web runs.
This system, which is based on the condition of having centering errors always of the same sign, and which can thus be corrected by a simple electromagnet, has however the drawback of limited accuracy. In practice, there is continuous oscillation of the effective cutting line about its ideal position, thus always determining a certain centering error, even though minimal, for each portion.
In addition, in devices of the described type, the adjustment errors must be detected in proximity to the cutting line, and preferably not more than one pitch upstream of the cutting means. If not, then because of the pitch variations between one motif and the next, and thus between the reference marks, any adjustment operation which is carried out a certain number of pitches or portions upstream of the cutting means can give rise to an incorrect centering of the printed motifs on the individual portions. In other words, under such operating conditions, the known described device detects a centering error relative to a certain reference mark but makes its correction relative to a reference mark which is different from the former.